Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), WiFi transceivers, and other communication devices transmit and receive communication signal at various frequencies that correspond to different communication bands and at varying power levels. Each of these devices uses a power amplifier to amplify the information signal for over-the-air transmission. One such power amplifier topology is referred to a collector voltage amplitude controller (COVAC). One technology used to implement a COVAC power amplifier uses one or more bipolar junction transistor (BJT) or heterojunction bipolar transistor (HBT) stages to implement the power amplifier, while the supply voltage is provided to the collector of the power amplifier using control circuitry that can be fabricated using a complementary metal oxide semiconductor (CMOS) process.
A typical COVAC power amplifier implementation can be used to transmit a signal that is modulated using Gaussian mean shift keying (GMSK) as the transmit methodology. GMSK power amplifiers are quite prevalent in portable handheld communication devices. The control circuitry associated with such a power amplifier typically receives a power control signal that is provided from an external source, such as a baseband system that is coupled to the power amplifier. The power output of the power amplifier is proportional to the level of the power control signal.
In general, it is desirable that the relationship between the power control signal and the power output of the power amplifier be linear, and under many operating conditions, this is the case. However, in some operating conditions, it is possible for the relationship between the power control signal and the power output of the power amplifier to become non-linear. One such operating condition is when the power amplifier becomes saturated. As an example, the power amplifier can become saturated under low battery conditions, or if a relatively high voltage standing wave ratio (VSWR) exists at the output of the power amplifier. When saturated, the power amplifier's internal power control loop gain-bandwidth decreases, resulting in high level switching transients on the falling edge of a transmit burst. When the power amplifier becomes saturated, it no longer responds in a linear manner to the power control signal.
Therefore, it would be desirable to detect the point at which a power amplifier becomes saturated, and to correct for such saturation.